The receptor with high affinity for IgE (FcepsilonRI) is a tetrameric complex of an IgE binding alpha chain, a beta chain and a dimer of gamma chains. In the past we have found that the extracellular region of the alpha chain is solely responsible for ligand binding. In order to further characterize the receptor binding site by crystallography, we have generated various transfectants capable of secreting large amounts of truncated ` chain capable of binding IgE. However this material, like the native ` is heavily glycosylated. This is preventing the formation of analyzable crystals. In order to circumvent this problem, we are following two different strategies: (1) The transfection of truncated alpha chain in cells with a defective glycosylation machinery. (2) The mutation of the seven N- linked glycosylation sites. Finally we are also attempting to produce a similar molecule in E. coli with the hope of using the NMR techniques to charaterize the receptor binding site.